Micromechanical structural elements having a self-supporting diaphragm may be used, for example, as sensor components of pressure sensors.
Sensors represent an additional application possibility, known from practical use, for a structural element of the type named at the outset, that are based on thermal effects, such as mass flow sensors, adiabatic gas heat dissipation sensors, thermal angle of inclination sensors, heat conductivity gas sensors or thermal infrared detectors. The sensor components of such sensors include, as a rule, temperature sensors and/or heaters which should be, to the greatest extent possible, thermally decoupled from their surroundings. The better the thermal decoupling, the slighter are the heat losses, and therewith the energy use, and the greater is the sensor's sensitivity. The self-supporting diaphragm, of the micromechanical structural element under discussion here, is used in these applications as a thermally insulated region. In order to minimize the heat dissipation via the substrate, and to hold the heat capacity of the diaphragm to a low value, the diaphragm should be designed as thin as possible. Besides that, it has proven advantageous, in this connection, to implement the diaphragm as made of a dielectric material, having a very slight heat conductivity and low heat capacity.
German published patent application no. 195 27 861 refers to a micromechanical structural element having a self-supporting diaphragm which is used as a sensor component for a mass flow sensor. The production of this sensor component starts from a silicon substrate on whose upper side a diaphragm layer is deposited. Heating elements and temperature sensors in the form of circuit board conductors are then generated on the diaphragm layer. The laying bare of the diaphragm is performed in a separate process step, in which, starting from the back, a cavity is etched into the silicon substrate. This back process is very time-intensive and critical with respect to yield, because one has to etch all the way through the entire thickness of the silicon substrate. The structural elements thus created are also very fragile.
In German patent application document no. 103 05 442, filed Feb. 11, 2003, a micromechanical structural element is discussed and described that has a self-supporting diaphragm, which can be produced in a purely front process. To do this, a diaphragm layer is deposited on a substrate. In order to bare the diaphragm, holes are produced in the diaphragm layer. Then, starting from the front side, the substrate material in the region under the holes is removed in an isotropic etching step, the substrate being able to be designated as a sacrificial layer down to the depth of the cavity produced thereby. A structuring of the back of the substrate in order to lay bare the diaphragm is not required in this instance.